A childhood friend of Woolwich murder suspect Michael Adebolajo posted a string of incendiary Twitter messages before his arrest, which followed an interview he gave to the BBC on Friday.
Abu Nusaybah was held by counter-terrorism officers at Broadcasting House in central London after giving an interview to BBC Newsnight in which he claimed MI5 had attempted to recruit his friend. He said his friend had told him he had been arrested, tortured and sexually abused while in custody in Kenya.
Nusaybah's arrest came before the interview was aired.
But Nusaybah had been posting updates on his Twitter account all week, prompted by events as they unfolded on the streets of Woolwich last Wednesday.
Suspects Michael Adebolajo and Michael Adebowale are both under armed guard in hospital after they were shot by police at the scene of Drummer Lee Rigby's murder.
But before speaking to the BBC, Nusaybah had posted a string of startling claims on Twitter.
"Did u know Woolwich suspect Michael Adebolajo was approached by MI5 Just over 6 months ago to work as a Spy, He refused?" he wrote.
He then alleged the UK government had knowledge of Adebolajo's torture and sexual abuse by Kenyan authorities. He said Adebolajo had told him this six months ago.
"Did U know Woolwich suspect Michael Adebolajo was arrested & tortured & sexually.... by Kenya Gov with UK gov knowledge?"
One hour later, he added: "I know this because Michael Adebolajo told me himself 6months ago."
It is not yet known whether it was these claims which led the authorites to arrest him.
Days earlier, Nusaybah took to Twitter to identify the Woolwich suspect as his old friend Adebolajo, who went by the nom de guerre "Mujahid", an Arabic word which denotes a Muslim engaging in Jihad.
Some of Nusaybah's messages of 22-23 May appeared to have been removed on the morning of 25 May, though it was not clear who was behind the removal.
On 23 May, the day after Rigby's killing, Nusaybah attempted to provide some insight into Adebolajo's motives.
"Mujahid was a strong character, but i remembers years back Foreign Policy would bring him to tears - then he just went quite (sic)," read a post on the morning after the attack.
Later that day, he wrote: "Muslims are angry about Afghanistan, Iraq, Drones, Guantanamo, Bagram, Rendition, Torture, False Intelligence, Where are the Comm Leaders?"
And later: "I can talk openly because my conscious is clear, I didn't know anything about this incident - had I know I would have spoken to him."
Nusaybah had known Adebolajo since they grew up together in Romford, Essex.
In a reference to major news outlets in the US and UK, Nusyabah added: "Plz make Dua / Supplication for all the Weak & Oppressed around the World who's pain has no voice from BBC, Sky, CNN, CBS, WSJ, NYT."
Referring to a speech in which President Barack Obama suggested the US may curtail its use of drones, Nusaybah wrote : "Some say it's rude to interrupt the president, but it's rude to kill innocent people with drones."
Nusaybah is believed to have previously acted as a spokesman for Waltham Forest Muslims, a group linked to the radical Islamist organisation al-Muhajiroun, and its subsequent reincarnation, Islam4UK.
That group was founded by Sheikh Omar Bakri Muhammad, the radical cleric said to have had close links to Osama bin Laden.
Bakri Muhammad was later extradited from the UK by then home secretary Charles Clarke, on the grounds his presence was "not conducive to the public good".
MI5 have confirmed that both suspects in the Woolwich murder were known to authorites, but said they were believed to be peripheral figures presenting no imminent threat.
TV footage has emerged from 2007 showing another radical Islamist, Anjem Choudary, with Adebolajo at a central London demonstration. Choudary took over as leader of the group after Bakri was refused permission to re-enter the UK.
Residents in Woolwich have also reported seeing Adebolajo in the town centre, making speeches and handing out leaflets.
In the BBC interview, Abu Nusaybah attributed Adebolajo's radicalisation to his purported abuse in Kenya.
Nusaybah alleged that Adebolajo was arrested while studying in a village in Kenya last year, and was then sexually assaulted.
On his return, "he became more reclined [sic], less talkative. He wasn't his bubbly self," said Nusaybah.
It has emerged that Adebolajo then went to a lawyer to complain of harassment and bugging by MI5, who have come under criticism for not recognising the threat he presented.
Whitehall sources were aware that Adebolajo was detained in Kenya before being deported, according to reports.
Asked about the unusual sequence of events leading to the arrest, the Metropolitan Police said: "He was not arrested because of his comments on Newsnight."
Scotland Yard said in a statement: "At around 2130 hours on Friday 24 May, a 31-year-old man was arrested in London by officers from the MPS Counter-Terrorism Command on suspicion of the commission, preparation or instigation of acts of terrorism under the Terrorism Act 2000.
"The man has been taken to a south London police station, where he remains in custody. Search warrants are being executed at two residential addresses in east London."